A typical EDM pulse supply in conventional use comprises a DC source and on-off controllable power switch, e.g. a bank of transistors, energized by signal pulses to periodically switch the DC source and to provide a sequence of EDM pulses of a preset duration .tau.on and a preset interval .tau.off across the gap formed between a tool electrode and a workpiece in the presence of a dielectric machining medium. The DC source comprises an input for receiving a single- or polyphase commercial AC supply and a transformer for transforming the commercial voltage to a desired machining voltage level. A rectifier is connected to provide a continuous and smoothened DC output from the transformed AC voltage. Machining power control may be effected by providing a saturable reactor in the AC circuit. The transformer and the saturable reactor operating at the commercial or low-frequency (i.e. 50 to 60 Hz) are large in capacity and size, large in power loss and also slow in response.
An improved EDM pulse supply makes use of a converter circuit which includes a rectifier directly connected to the AC supply to directly provide a DC output which is switched by a power switch to provide a sequence of high-frequency voltage pulses. A high-frequency transformer is included in the converter and can advantageously be small in size. Machining power control can simply be effected by adjusting the operating frequency of the converter, permitting the power unit to be compact and extremely small sized and affording a greatly reduced power loss. The high-frequency transformer has its output winding connected to the machining gap via a rectifies to apply a sequence of unidirectional machining pulses of a desired polarity and voltage level. A further power switch may be connected in the gap discharge circuit and can be periodically turned on and off to provide a sequence of machining pulses whose duration .tau.on and interval .tau.off are preset independently of the pulses which develop at the input side of the high-frequency transformer.
It is a critical requirement in a given EDM operation that each individual machining electrical discharge be strictly set as to its duration .tau.on and interval .tau.off in conformity with the tool and workpiece materials and other machining conditions and in accordance with the desired end results of the operation, e.g. the surface finish, removal rate and overcut. The duration .tau.on and interval .tau.off can be preset with each signal pulse produced in the switching control circuit for the power switch connected in circuit with the gap discharge circuit. It has been entirely common, therefore, in the prior art, regardless of different types of the power switching EDM pulse supply, to furnish the power switch with a sequence of switching signal pulses so that the DC source is switched on and each machining discharge occurs upon energization of the switch by each signal pulse and terminates upon termination of the signal pulse. Signal pulses are strictly preset as to their individual duration which determines the duration .tau.on of individual machining discharge pulses. Since each switching signal pulse is preset and dimensioned to precisely determine the duration .tau.on of each individual discharge, it is common knowledge that the switching and gap discharge circuit should be devoid of any impedance means, i.e. inductor and capacitor, including a stray inductance and capacitance, which tends to deform or distort the machining pulses. Thus, in every prior EDM power supply in which a sequence of machining discharge pulses are provided by the switching of a power switch, it has been commonly recognized to be essential that the switching be so sharp and the gap discharge circuit be so free from impedance means that each discharge pulse sharply develops and terminates, because if this fails, the duration of discharge pulses would undesirably deviate from the preset value. Such failure has also been believed to be disadvantageous since successive pulses tend to result in a continuous arcing--rather than be discrete as is essential--which would damage the machining operation.
In addition to the foregoing common knowledge of designing the pulsing circuitry in the prior EDM art, it should be pointed out that it is desirable that an EDM pulse supply unit be capable of providing various modified pulse formats to meet a wide ranges of machining requirements. For example, a pulse format has been known to be desirable which comprises a sequence of successive time-spaced groups of discrete, time-spaced machining discharge pulses. The number of machining pulses included in each train and the time interval between successive trains, together with the duration and/or interval of the machining pulses are preset to meet the particular requirement and may, together with or without a change of the duration and/or interval of the machining pulses, be varied in accordance with the machining gap condition or with the progress of the machining operation. A further modified pulse format, which has also be found to improve EDM processes, incorporates periodically a pulse of greater energy among a train of machining pulses of a preset energy or successive time-spaced trains of such machining pulses. The timing and energy parameters of such an energy-increased pulse may be controlled in response to the gap condition and/or in accordance with the machining progress. Furthermore, fine and ultra-fine EDM operatio require discharge pulses which in duration are narrow or ultra-narrow and yet are discrete and time-spaced. Here again, the duration and interval of such pulses should desirably be variably preset depending on particular machining settings and controlled with the machining progress. It can be seen, therefore, that a pressing demand exists for an EDM power supply which is fully versatile as to the pulse settability and controllability.